Pump Unit And Hydrostatic Transmission

ABSTRACT

There is provided a pump unit configured to be capable of transmitting a rotational per from a driving power source which is operatively connected thereto to an actuator through a pair of hydraulic fluid lines. The pump unit includes: an input shaft; a hydraulic pump main body; a PTO shaft; a PTO clutch mechanism; and auxiliary pump main body; a suction line; a discharge line; a charge line; a PTO hydraulic fluid line; a pressure-reducing valve; and a resistance valve. The PTO hydraulic fluid line and the charge line are fluidly connected to the primary side and a secondary side of the pressure-reducing valve, respectively.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/206,451, filed Sep. 8, 2008, which is a continuation of U.S.application Ser. No. 11/746,945, filed May 10, 2007, which is adivisional of U.S. application Ser. No. 11/269,643, filed Nov. 9, 2005,the entire disclosures of which are hereby incorporated by referencethereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a pump unit and a hydrostatictransmission.

2. Related Art

It has been conventionally known that, in pump units configured to becapable of transmitting a rotational power from a driving power sourcethat is operatively connected thereto to an actuator through a pair ofhydraulic fluid lines, and in hydrostatic transmissions (hereinafter,also referred to HST) which non-stepwisely change the speed of arotational power from a driving power source which is operativelyconnected thereto by using the hydraulic pressure of hydraulic fluidflowing through a pair of hydraulic fluid lines, there is provided a PTOtransmission mechanism for branching and outputting the power from thedriving power source.

In pump units or hydrostatic transmissions as described above, there maybe provided an auxiliary pump main body which is operatively driven bythe driving power source in order to supply hydraulic fluid forreplenishing to the pair of hydraulic fluid lines and supply hydraulicfluid for operating to the PTO clutch mechanism provided in the PTOtransmission mechanism (refer to, for example, JP-A 2003-291674).

A conventional pump unit or conventional hydrostatic transmissionincludes a suction line for fluidly communicating between a suction portof the auxiliary pump main body and a fluid reservoir, a discharge linewhich is fluidly connected to a discharge port of the auxiliary pumpmain body, a charge line which is branched from the discharge line via apressure-reducing valve for supplying hydraulic fluid for replenishingto the pair of hydraulic fluid lines, and a PTO hydraulic fluid linewhich is branched from the discharge line via the pressure-reducingvalve for supplying hydraulic fluid for operating to the aforementionedPTO clutch mechanism.

In the conventional pump unit or the hydrostatic transmission, thecharge line is fluidly connected to a secondary side of thepressure-reducing valve and the PTO hydraulic fluid line is fluidlyconnected to a spring chamber of the pressure-reducing valve.Consequently, when the PTO clutch mechanism is engaged or disengaged,the hydraulic pressure in the charge line is fluctuated, thus causingthe malfunction of fluctuations in the operation torque of theoutput-adjusting member used for changing and operating thesuction/discharge rate of the hydraulic pump main body.

Namely, the pressure-reducing valve is configured to be capable ofdefining the hydraulic pressure in the charge line to a predeterminedhydraulic pressure with a hydraulic pressure setting spring placedwithin the spring chamber and, as previously described, a conventionalpump unit or hydrostatic transmission is configured such that the PTOhydraulic fluid line is supplied with hydraulic fluid from the springchamber of the pressure-reducing valve.

In the PTO hydraulic fluid line, there is inserted a PTO relief valvefor setting the hydraulic pressure in the PTO clutch mechanism, inaddition to a switching valve for switching between supply andinterruption of hydraulic fluid to the PTO clutch mechanism. Therefore,when the PTO clutch mechanism is disengaged, the PTO hydraulic fluidline is at a natural pressure that is communicated with the fluidreservoir, while when the PTO clutch mechanism is engaged, it is set toan operational pressure defined by the PTO relief valve.

Consequently, within the spring chamber of the pressure-reducing valveconnected to the PTO hydraulic fluid line, the hydraulic pressure isvaried along with the hydraulic pressure fluctuation in the PTOhydraulic fluid line. Thus, the hydraulic pressure in the charge line,which is defined by the hydraulic pressure setting spring within thespring chamber, is also varied along with the hydraulic pressurefluctuation in the PTO hydraulic fluid line.

As described above, with conventional pump units and hydrostatictransmissions, there have been caused malfunctions of fluctuation in theoperation torque of the output-adjusting member of the hydraulic pumpmain body due to the difference of the hydraulic pressure in the chargeline between when the PTO clutch mechanism is engaged and when it isdisengaged.

Also, in order to gradually raise the hydraulic pressure in the PTOhydraulic fluid line to the hydraulic pressure defined by the PTO reliefvalve for preventing abrupt engaging movement of the PTO clutchmechanism, an accumulator may be inserted in the PTO hydraulic fluidline, in some cases.

However, sufficient considerations have not been given to the placementof such an accumulator in conventional pump units and hydrostatictransmissions.

Also, pump units as described above may include a pair of hydraulic pumpmain bodies. Such pump units are configured to be capable oftransmitting a rotational power from a driving power source which isoperatively connected thereto to first and second hydraulic motor unitsfor driving the left and right driving wheels through a pair of firsthydraulic fluid lines and a pair of second hydraulic fluid lines.

Namely, in the pump unit, the pair of hydraulic pump main bodies and thepair of hydraulic motor units form a pair of HSTs for individuallydriving the pair of left and right driving wheels.

However, with a conventional pump unit including such a pair ofhydraulic pump main bodies, even though the respective output-adjustingmembers in the pair of HSTs are operated such that the pair of hydraulicmotor units are brought into the same output state, there may occuroutput differences between the pair of hydraulic motor units due tofabrication errors and assembly errors.

Such output differences between the pair of hydraulic motor units maycause a so-called wandering phenomenon that the vehicle meanders eventhough the vehicle is manipulated such that it linearly moves.

The present invention was made in view of the aforementioned prior art.It is an object of the present invention to provide pump units orhydrostatic transmissions including a PTO clutch mechanism, the pumpunits or hydrostatic transmission being capable of maintaining thecharge pressure for a pair of hydraulic fluid lines constant, regardlessof whether the PTO clutch mechanism is engaged or disengaged.

It is another object of the present invention to provide pump units orhydrostatic transmissions including an accumulator for gradually raisingthe hydraulic pressure of hydraulic fluid for the PTO clutch mechanism,the pump units or hydrostatic transmissions being capable of preventingthe increase of the apparatus size and also enhancing a pipingworkability for the accumulator.

It is still another object of the present invention to provide a pumpunit for vehicle-traveling configured to include a pair of hydraulicpump main bodies which are operatively connected to a driving powersource and transmit a rotational power from the driving power source tofirst and second hydraulic motor units for driving right and leftdriving wheels, through a pair of first hydraulic fluid lines and a pairof second hydraulic fluid lines, the pump units being capable ofindividually driving the right and left driving wheels in avariable-speed manner and effectively preventing the occurrence ofwandering phenomena during straight-movement operation of the vehicle.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided apump unit configured to be capable of transmitting a rotational powerfrom a driving power source which is operatively connected thereto to anactuator through a pair of hydraulic fluid lines.

The pump unit includes: an input shaft which is operatively connected tothe driving power source; a hydraulic pump main body which isoperatively driven by the input shaft and is fluidly connected to theactuator through the pair of hydraulic fluid lines; a PTO shaft which isoperatively driven by the input shaft; a PTO clutch mechanism which isinserted in a transmission path from the input shaft to the PTO shaft;an auxiliary pump main body; a suction line which has a first endfluidly connected to a fluid reservoir and a second end fluidlyconnected to a suction port of the auxiliary pump main body; a dischargeline which is fluidly connected to the discharge port of the auxiliarypump main body; a charge line which supplies a part of hydraulic fluidfrom the discharge line to the pair of hydraulic fluid lines throughcheck valves; a PTO hydraulic fluid line which supplies a part ofhydraulic fluid from the discharge line to the PTO clutch mechanismthrough a PTO relief valve; a pressure-reducing valve which has aprimary side fluidly connected to the discharge line; and a resistancevalve which is fluidly connected to the primary side of thepressure-reducing valve.

In the pump unit, the PTO hydraulic fluid line and the charge line arefluidly connected to the primary side and a secondary side of thepressure-reducing valve, respectively.

With this configuration, it is possible to maintain the hydraulicpressure in the charge line at the charge pressure defined by thepressure-reducing valve, without affected by hydraulic pressurefluctuations in the PTO hydraulic fluid line.

Therefore, the charge pressure for the pair of hydraulic fluid linescould be maintained regardless of whether the PTO clutch mechanism isengaged or disengaged, thereby the operation torque of theoutput-adjusting member being constant.

Preferably, the pump unit further includes a drain line which fluidlyconnects a secondary side of the resistance valve to the fluidreservoir. The drain line is so configured to have an oil coolerinserted therein.

In one embodiment, the pump unit may include a drain line which fluidlyconnects the secondary side of the resistance valve to the fluidreservoir; and an external hydraulic fluid extraction line which isbranched off from the drain line.

In the one embodiment, preferably, the pump unit further includes anexternal hydraulic pressure relief valve which has a primary sidefluidly connected to the discharge line. The hydraulic pressure in theexternal hydraulic fluid extraction line is set by the externalhydraulic pressure relief valve.

More preferably, the pump unit further includes a return line which hasa first end fluidly connected to the discharge line and a second endfluidly connected to the suction line. The external hydraulic pressurerelief valve is inserted in the return line.

In the above various configurations, the PTO hydraulic fluid line may befluidly connected to the primary side of the pressure-reducing valvethrough an orifice.

According to one aspect of the present invention, there is also provideda hydrostatic transmission for vehicle traveling, which non-stepwiselychanges a speed of a rotational power from a driving power source whichis operatively connected thereto by means of hydraulic actions.

The hydrostatic transmission includes: an input shaft which isoperatively connected to the driving power source; a hydraulic pump mainbody which is operatively driven by the input shaft; a hydraulic motormain body which is fluidly connected to the hydraulic pump main body; amotor shaft which is rotated and driven by the hydraulic motor mainbody, and operatively connected to a driving wheel; a PTO shaft which isoperatively driven by the input shaft; a PTO clutch mechanism which isinserted in a transmission path from the input shaft to the PTO shaft;an auxiliary pump main body; a pair of hydraulic fluid lines whichfluidly connects the hydraulic pump main body to the hydraulic motormain body; a suction line which has a first end fluidly connected to afluid reservoir and a second end fluidly connected to a suction port ofthe auxiliary pump main body; a discharge line which is fluidlyconnected to a discharge port of the auxiliary pump main body; a chargeline which supplies a part of hydraulic fluid from the discharge line tothe pair of hydraulic fluid lines through check valves; a PTO hydraulicfluid line which supplies a part of hydraulic fluid from the dischargeline to the PTO clutch mechanism through a PTO relief valve; apressure-reducing valve which has a primary side fluidly connected tothe discharge line; and a resistance valve which is fluidly connected tothe primary side of the pressure-reducing valve.

In the thus configured pump unit, the PTO hydraulic fluid line and thecharge line are fluidly connected to the primary side and a secondaryside of the pressure-reducing valve, respectively.

With this configuration, it is possible to maintain the hydraulicpressure in the charge line at the charge pressure defined by thepressure-reducing valve, without affected by hydraulic pressurefluctuations in the PTO hydraulic fluid line.

Therefore, the charge pressure for the pair of hydraulic fluid linescould be maintained regardless of whether the PTO clutch mechanism isengaged or disengaged, thereby the operation torque of theoutput-adjusting member being constant.

Preferably, the hydrostatic transmission may further include a drainline which fluidly connects a secondary side of the resistance valve tothe fluid reservoir. The drain line is so configured to have an oilcooler inserted therein.

In one embodiment, the hydrostatic transmission further includes a drainline which fluidly connects the secondary side of the resistance valveto the fluid reservoir; and an external hydraulic fluid extraction linewhich is branched off from the drain line.

In the one embodiment, preferably, the hydrostatic transmission furtherincludes an external hydraulic pressure relief valve which has a primaryside fluidly connected to the discharge line. The hydraulic pressure inthe external hydraulic fluid extraction line is set by the externalhydraulic pressure relief valve.

In the one embodiment, more preferably, the hydrostatic transmissionfurther includes a return line which has a first end fluidly connectedto the discharge line and a second end connected to the suction line.The external hydraulic pressure relief valve is inserted in the returnline.

In the above various configurations, preferably, the PTO hydraulic fluidline may be fluidly connected to the primary side of thepressure-reducing valve through an orifice.

According to another aspect of the present invention, there is provideda pump unit configured to be capable of transmitting a rotational powerfrom a driving power source which is operatively connected thereto to anactuator through a pair of hydraulic fluid lines.

The pump unit includes: an input shaft which is operatively connected tothe driving power source; a hydraulic pump main body which isoperatively driven by the input shaft and is fluidly connected to theactuator through the pair of hydraulic fluid lines; a PTO shaft which isoperatively driven by the input shaft; a PTO clutch mechanism which isinserted in a transmission path from the input shaft to the PTO shaft;an auxiliary pump main body; a housing which accommodates the hydraulicpump main body and the PTO clutch mechanism; a discharge line which isfluidly connected to a discharge port of the auxiliary pump main body; aPTO hydraulic fluid line which is fluidly connected to the dischargeline and supplies hydraulic fluid to the PTO clutch mechanism; a PTOrelief valve which sets the hydraulic pressure in the PTO hydraulicfluid line; and an accumulator which is inserted in the PTO hydraulicfluid line and raises the hydraulic pressure in the PTO hydraulic fluidline to the pressure set by the PTO relief valve. The accumulator isincorporated in the housing.

With this configuration, it is possible to enhance the efficiency ofpiping operation and prevent leakage of PTO hydraulic fluid withreducing the size of the entire apparatus.

For example, the housing includes: a housing main body which is providedwith an opening for allowing the hydraulic pump main body to passtherethrough, the opening being provided at one side in the direction ofa rotation axis of the hydraulic pump main body; and a port block whichis detachably connected to the housing main body such that it abuts thehydraulic pump main body and closes the opening of the housing main bodyin a liquid-tight manner, the port block being provided with a pair ofhydraulic fluid passages forming a part of the pair of hydraulic fluidlines.

The housing main body includes a first end-face portion provided withthe opening, a second end-face portion positioned at the opposite sidefrom the first end-face portion with respect to the direction of therotation axis of the hydraulic pump main body, and a wall portionextending in the direction of the rotation axis of the hydraulic pumpmain body between the first end-face portion and the second end-faceportion.

In one embodiment, the accumulator may be incorporated within the wallportion such that its axis is along the direction of the rotation axisof the hydraulic pump main body.

In another embodiment, the accumulator is incorporated within the firstend-face portion or the second end-face portion such that its axis issubstantially orthogonal to the direction of the rotation axis of thehydraulic pump main body.

According to another aspect of the present invention, there is provideda hydrostatic transmission which non-stepwisely changes a speed of arotational power from a driving power source which is operativelyconnected thereto by means of hydraulic actions.

The hydrostatic transmission includes: an input shaft which isoperatively connected to the driving power source; a hydraulic pump mainbody which is operatively driven by the input shaft; a hydraulic motormain body which is fluidly connected to the hydraulic pump main body; amotor shaft which is rotated and driven by the hydraulic motor mainbody; a PTO shaft which is operatively driven by the input shaft; a PTOclutch mechanism which is inserted in a transmission path from the inputshaft to the PTO shaft; an auxiliary pump main body; a housing whichaccommodates the hydraulic pump main body, the hydraulic motor main bodyand the PTO clutch mechanism; a discharge line which is fluidlyconnected to a discharge port of the auxiliary pump main body; a PTOhydraulic fluid line which is fluidly connected to the discharge lineand supplies hydraulic fluid to the PTO clutch mechanism; a PTO reliefvalve which sets the hydraulic pressure in the PTO hydraulic fluid line;and an accumulator which is inserted in the PTO hydraulic fluid line andgradually raises the hydraulic pressure in the PTO hydraulic fluid lineto the pressure set by the PTO relief valve. The accumulator isincorporated in the housing.

With this configuration, it is possible to enhance the efficiency ofpiping operation and prevent leakage of PTO hydraulic fluid withreducing the size of the entire apparatus.

For example, the housing includes: a housing main body which is providedwith an opening for allowing the hydraulic pump main body and thehydraulic motor main body to pass therethrough, the opening beingprovided at one side in the direction of the rotation axis of thehydraulic pump main body; and a port block which is detachably connectedto the housing main body such that it abuts the hydraulic pump main bodyand the hydraulic motor main body, and closes the opening of the housingmain body in a liquid-tight manner, the port block being provided with apair of hydraulic fluid passages which fluidly connect the hydraulicpump main body and the hydraulic motor main body to each other.

The housing main body includes a first end-face portion provided withthe opening, a second end-face portion positioned at the opposite sidefrom the first end-face portion with respect to the direction of therotation axis of the hydraulic pump main body and a wall portionextending in the direction of the rotation axis of the hydraulic pumpmain body between the first end-face portion and the second end-faceportion.

In one embodiment, the accumulator may be incorporated within the wallportion such that its axis is along the direction of the rotation axisof the hydraulic pump main body.

In another embodiment, the accumulator is incorporated within the firstend-face portion or the second end-face portion such that its axis issubstantially orthogonal to the direction of the rotation axis of thehydraulic pump main body.

According to still another aspect of the present invention, there isprovided a pump unit for vehicle traveling configured to be capable oftransmitting a rotational power through a pair of first hydraulic fluidlines and a pair of second hydraulic fluid lines from a driving powersource which is operatively connected thereto to first and secondhydraulic motor units for driving right and left driving wheels.

The pump unit includes: an input shaft which is operatively connected tothe driving power source; first and second hydraulic pump main bodieswhich are operatively driven by the input shaft and are fluidlyconnected to the first and second motor units through the pair of firsthydraulic fluid lines and the pair of second hydraulic fluid lines; ahousing main body which surrounds the first and second hydraulic pumpmain bodies; a port block which abuts and supports the first and secondhydraulic pump main bodies, and defines the accommodating space for thefirst and second hydraulic pump main bodies in cooperation with thehousing main body; and a communication line which communicates between aforward-movement high-pressure side first hydraulic fluid line, which isbrought into higher pressure during forward movement, out of the pair offirst hydraulic fluid lines and a forward-movement high-pressure sidesecond hydraulic fluid line, which is brought into higher pressureduring forward movement, out of the pair of second hydraulic fluidlines, the communication line having an orifice inserted therein.

With this configuration, it is possible to maintain the individualtransmissions of the first HST formed by the first hydraulic pump mainbody and the first hydraulic motor unit and the second HST formed by thesecond hydraulic pump main body and the second hydraulic motor unit,while effectively preventing the occurrence of wandering phenomena thatthe vehicle meanders even though the first HST and the second HST areoperated such that they are brought into the same output state formoving the vehicle straightly.

Preferably, the port block is provided with a pair of first hydraulicfluid passages and a pair of second hydraulic fluid passages which forma part of the pair of first hydraulic fluid lines and the pair of secondhydraulic fluid lines, respectively, and the communication line isconfigured to communicate between a forward-movement high-pressure sidefirst hydraulic fluid passage, which is brought into higher pressureduring forward movement, out of the pair of first hydraulic fluidpassages and a forward-movement high-pressure side second hydraulicfluid passage, which is brought into higher pressure during forwardmovement, out of the pair of second hydraulic fluid passages.

In one embodiment, the pair of first hydraulic fluid passages and thepair of second hydraulic fluid passages are formed to be substantiallyparallel to each other, and the communication line has a communicationfluid passage formed in the port block such that it is orthogonal to thepair of first hydraulic fluid passages and the pair of second hydraulicfluid passages.

Preferably, the forward-movement high-pressure side first hydraulicfluid passage and the forward-movement high-pressure side secondhydraulic fluid passage are placed adjacent to each other.

In another embodiment, the communication line has a conduit which isdetachably connected to the port block such that it communicates theforward-movement high-pressure side first hydraulic fluid passage andthe forward-movement high-pressure side second hydraulic fluid passageto each other.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, and other objects, features and advantages of the presentinvention will become apparent from the detailed description thereof inconjunction with the accompanying drawings wherein.

FIG. 1 is an oil hydraulic circuit diagram of a working vehicle to whicha pump unit according to a first embodiment of the present invention isapplied.

FIG. 2 is a cross-sectional plan view of the pump unit of the firstembodiment.

FIG. 3 is a longitudinal cross-sectional side view of the pump unittaken along a line in FIG. 2.

FIG. 4 is a longitudinal cross-sectional side view of the pump unittaken along a line IV-IV in FIG. 2.

FIG. 5 is a cross-sectional view taken along a line V-V in FIG. 2.

FIG. 6 is a cross-sectional view taken along a line VI-VI in FIG. 2.

FIG. 7 is a cross-sectional view taken along a line VII-VII in FIG. 2.

FIG. 8 is a cross-sectional view taken along a line VIII-VIII in FIG. 2.

FIG. 9 is a cross-sectional view of a port block in a modified pump unitof the present invention.

FIG. 10 is a cross-sectional view of an auxiliary pump case, taken alonga line X-X in FIG. 2.

FIG. 11 is a cross-sectional view taken along a line XI-XI in FIG. 10.

FIG. 12 is a cross-sectional view taken along a line XII-XII in FIG. 10.

FIG. 13 is a cross-sectional view of a modified pump unit of the presentinvention.

FIG. 14 is a cross-sectional view of a port block in a modified pumpunit of the present invention.

FIG. 15 is a hydraulic circuit diagram of a vehicle to which a pump unitaccording to a second embodiment of the present invention.

FIG. 16 is a longitudinal cross-sectional front view of a port block inthe pump unit of the second embodiment.

FIG. 17 is a cross sectional view taken along a line XVII-XVII in FIG.16.

FIG. 18 is a longitudinal cross-sectional view of a port block in amodified pump unit of the present invention.

FIG. 19 is a longitudinal cross-sectional view of a port block inanother modified pump unit of the present invention.

FIG. 20 is a longitudinal cross-sectional view of a port block in stillanother modified pump unit of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

Hereinafter, a preferred embodiment of a pump unit according to thepresent invention will be described with reference to the attacheddrawings.

FIG. 1 illustrates an oil hydraulic circuit diagram of a working vehicle1 to which a pump unit according to this embodiment is applied. FIG. 2illustrates a cross-sectional plan view of the pump unit 100 accordingto this embodiment. FIG. 3 and FIG. 4 are longitudinal cross-sectionalside views of the pump unit 100 taken along a line and a line IV-IV inFIG. 2, respectively.

The pump unit 100 according to this embodiment is configured to form amain transmission path for transmitting the rotational power from adriving power source (not shown) to an actuator through a pair ofhydraulic fluid lines 400 a and 400 b and also is configured to form asecondary transmission path for diverging and outputting the rotationalpower from the driving power source to the outside.

More specifically, as illustrated in FIG. 1 to FIG. 4, the pump unit 100includes an input shaft 220 which is operatively connected to thedriving power source, hydraulic pump main bodies 300 a and 300 b whichare operatively driven by the input shaft 220 and are fluidly connectedto the aforementioned actuator through the pair of hydraulic fluid lines400 a and 400 b, a PTO shaft 610 which is operatively driven by theinput shaft 220, and a PTO clutch mechanism 600A inserted in thetransmission path from the input shaft 200 to the PTO shaft 610, whereinthe hydraulic pump main bodies 300 a and 300 b form a part of the maintransmission path while the PTO shaft 610 and the PTO clutch mechanism600A form a part of the secondary transmission path.

Further, the pump unit 100 according to this embodiment includes a PTObraking mechanism 600B which operates in conjunction with the PTO clutchmechanism 600A. The PTO braking mechanism 600B is provided in order toprevent the PTO shaft 610 from being continuously rotated due to aninertial force when the power to the PTO shaft 610 from the input shaft220 is shutoff.

In this embodiment, the pump unit 100 includes a pair of first andsecond hydraulic pump main bodies 300 a and 300 b as the aforementionedhydraulic pump main bodies.

The pair of first and second hydraulic pump main bodies 300 a and 300 bare fluidly connected to a pair of first and second hydraulic motorunits 10 a and 10 b which serve as the aforementioned actuator throughthe pair of first and second hydraulic fluid lines 400 a and 400 b,respectively. The first hydraulic pump main body 300 a and the firsthydraulic motor unit 10 a form a first HST for vehicle-traveling whichdrives one of a pair of driving wheels 50 with variable rotationalspeeds. The second hydraulic pump main body 300 b and the secondhydraulic motor unit 10 b form a second HST for vehicle-traveling whichdrives the other one of the pair of driving wheels 50 with variablerotational speeds.

Namely, out of the first hydraulic pump main body 300 a and the motormain body in the first hydraulic motor unit 10 a, at least one of themis of a variable displacement type.

Similarly, out of the second hydraulic pump main body 300 b and themotor main body in the second hydraulic motor unit 10 b, at least one ofthem is of a variable displacement type.

In this embodiment, the first and second hydraulic pump main bodies 300a and 300 b are of a variable displacement type while the first andsecond hydraulic motor units 10 a and 10 a are of a fixed displacementtype.

More specifically, the pump unit 100 includes a housing 200 whichsupports the input shaft 220 and the PTO shaft 610 and accommodates thefirst and second hydraulic pump main bodies 300 a and 300 b and the PTOclutch mechanism 600A, and a transmission mechanism 280 which transmitsthe power from the input shaft 220 to the first hydraulic pump main body300 a, the second hydraulic pump main body 300 b and the PTO clutchmechanism 600A and also is accommodated within the housing 200, inaddition to the input shaft 200, the first and second hydraulic pumpmain bodies 300 a and 300 b, the PTO shaft 610 and the PTO clutchmechanism 600A.

FIG. 5 to FIG. 8 illustrate cross-sectional views taken along a lineV-V, a line VI-VI, a line VII-VII and a line VIII-VIII in FIG. 2,respectively.

As illustrated in FIG. 2 and FIG. 5 to FIG. 7, the housing 200 includesa housing main body 210 which accommodates the first and secondhydraulic pump main bodies 300 a and 300 b and the PTO clutch mechanism600A, a center section (port block) 360 which is detachably connected tothe housing main body 210 and is provided with fluid passages fluidlycommunicated with the first and second hydraulic pump main bodies 300 aand 300 b, and a cover member 250 which is detachably connected to thehousing main body 210 at the opposite side from the port block 360 inthe direction of the rotation axes of the hydraulic pump main bodies 300a and 300 b.

In this embodiment, the first and second hydraulic pump main bodies 300a and 300 b are accommodated within the housing 200 such that thedirections of their rotation axes are in parallel with each other alongthe fore-to-aft direction of the vehicle. Further, the PTO shaft 610 isalso supported by the housing 200 such that the rotation axis thereof isin parallel with the direction of the rotation axes of the first andsecond hydraulic pump main bodies 300 a and 300 b.

More specifically, as illustrated in FIG. 2, the housing main body 210includes a first end-face portion 211 positioned at one side in thedirection of the rotation axes of the first and second hydraulic pumpmain bodies 300 a and 300 b, a second end-face portion 212 spaced apartfrom the first end-face portion 211 at the other side in the directionof the rotation axes, and a wall portion 213 extending along thedirection of the rotation axes between the first end-face portion 211and the second end-face portion 212.

The first end-face portion 211 includes a first opening 211 a whichallows the first and second hydraulic pump main bodies 300 a and 300 bto pass therethrough, at the one side in the widthwise direction of thehousing main body 210.

Further, the first opening 211 a is liquid-tightly sealed with the portblock 360.

Namely, the housing 200 is configured to define a pump accommodatingspace 201 for accommodating the first and second hydraulic pump mainbodies 300 a and 300 b between the port block 360 and the secondend-face portion 212 (see FIG. 2).

The second end-face portion 212 includes a swash plate receiving portion212 a having a concave arc-shaped guiding surface for slidably abuttingand supporting a movable swash plate 353 which will be described laterof the first and second hydraulic pump main bodies 300 a and 300 b atthe position corresponding to the pump accommodating space 201, and asecond opening 212 b which allows the PTO clutch mechanism 600A to passtherethrough at the other side of the housing 200 in thevehicle-widthwise direction (see FIG. 2).

Further, the second opening 212 b is liquid-tightly sealed with thecover member 250.

Namely, the housing 200 is configured to define a PTO clutchaccommodating space 202 for accommodating the PTO clutch mechanism 600Abetween the cover member 250 and the first end-face portion 211.

As described above, in this embodiment, the internal space of thehousing 200, which is defined by the port block 360 and the secondend-face portion 212, at one side in the widthwise direction is utilizedas the pump accommodating space 201 while the internal space of thehousing 200, which is defined by the cover member 250 and the firstend-face portion 211, at the other side in the widthwise direction isutilized as the PTO clutch accommodating space 202.

Further, in this embodiment, the swash plate receiving portion 212 a isformed integrally with the housing main body 210 and, accordingly, thefirst opening 211 a which allows the first and second hydraulic pumpmain bodies 300 a and 300 b to pass therethrough and the second opening212 b which allows the PTO clutch mechanism 600A to pass therethroughare provided at one side and the other side in the direction of therotation axes of the first and second hydraulic pump main bodies 300 aand 300 b. However, instead thereof, the swash plate receiving portion212 a may be formed separately from the housing main body 210.

By forming the swash plate receiving portion 212 a separately from thehousing main body 210, it is possible to eliminate the necessity of thesecond end-face portion 212. Consequently, when molding the housing mainbody 210, it is possible to pull out the respective molds for formingthe hydraulic pump housing portion 201 and the PTO clutch housingportion 202 in the same direction.

The cover member 250 is detachably connected to the housing main body210 so as to form a transmission mechanism accommodating space 203 (seeFIG. 2) for accommodating the transmission mechanism 280 between thecover member 250 and the second end-face portion 212 of the housing mainbody 210.

Further, as illustrated in FIG. 3, the second end-face portion 212 maybe provided with a communication channel 215 which allows fluid tofreely flow between the hydraulic pump accommodating space 201 and thetransmission mechanism accommodating space 203.

As illustrated in FIG. 2 and FIG. 3, the input shaft 220 is supported bythe housing 200 in a state where its first end at the upstream side inthe direction of transmission is outwardly extended from the housing 200so as to be operatively connected to the driving power source.

In this embodiment, the input shaft 200 is formed integrally with afirst pump shaft 310 a of the first hydraulic pump main body 300 a.

In this embodiment, the first hydraulic pump main body 300 a is of anaxial piston type as illustrated in FIG. 2 and FIG. 3.

More specifically, the first hydraulic pump main body 300 a includes thefirst pump shaft 310 a which is operatively connected to the input shaft220, a first cylinder block 320 a supported on the first pump shaft 310a in a relatively non-rotatable manner, and a first piston unit 330 aaccommodated within the cylinder block 320 a in a relativelynon-rotatable manner and a relatively axially slidable manner.

As previously described, in this embodiment, the first hydraulic pumpmain body 300 a is of a variable displacement type.

Consequently, in addition to the aforementioned structures, the firsthydraulic pump main body 300 a includes an output-adjusting member 340 afor changing the range of sliding of the piston unit 330 a to adjust thesuction/discharge rate.

In this embodiment, a cradle-type movable swash plate is employed as theoutput-adjusting member 340 a.

More specifically, the movable swash plate 340 a has a convex arc-shapedrear surface which is in intimate contact with a thrust metal secured onthe concave arc-shaped surface of the swash plate receiving portion 212a so that the movable swash plate 340 a is slidably supported, and afront surface which is abutted against a shoe provided at the tip end ofthe piston unit 330 a. Further, there is provided a thrust metalengaging protrusion 216 for preventing the inclination of the thrustmetal, at the deepest portion of the concave arc-shaped surface of theswash plate receiving portion 212 a (see FIG. 3).

Further, the output-adjusting member 340 a can be manipulated fromoutside using a control shaft 350 a which is connected thereto throughan arm 351 a (see FIG. 3).

As previously described, in this embodiment, a first side of the firstpump shaft 310 a is formed integrally with the input shaft 220 which isoutwardly extended.

Further, as illustrated in FIG. 2 and FIG. 3, a second side of the firstpump shaft 310 a is supported by the port block 360, the second end-faceportion 212 and the cover member 250 in a relatively rotatable mannerabout its axis, at a state where the second side at the downstream-sideend portion in the direction of transmission is outwardly extendedthrough the port block 360.

In this embodiment, a cooling fan 800 is provided on the second side ofthe first pump shaft 310 a (see FIG. 2).

Obviously, the outwardly extended portion of the first pump shaft 310 aat the second side may be used as the input shaft 220. Further, a secondpump shaft 310 b, which will be described later, may be configured tohave an outwardly extended portion beyond an auxiliary pump case 520,and then the cooling fan 800 may be provided on the outwardly extendedportion of the second pump shaft 310 b.

The second hydraulic pump maim body 300 b has substantially the samestructure as that of the first hydraulic pump maim body 300 a.

Consequently, detailed description of the second hydraulic pump maimbody 300 b will be properly omitted by replacing the end referencecharacter “a” of the first hydraulic pump maim body 300 a with “b”.

The second pump shaft 310 b is supported in a relatively rotatablemanner about its axis by the port block 360, the second end-face portion212 and the cover member 250 at a state where a downstream-side endportion in the direction of transmission is extended outwardly throughthe port block 360.

Further, an auxiliary pump main body 510, which will be described later,is supported on the outwardly extended portion of the second pump shaft310 b.

As illustrated in FIG. 2, the PTO shaft 610 is supported in a relativelyrotatable manner about its axis by the first end-face portion 211 andthe cover member 250 at a state where the downstream-side end portion inthe direction of transmission is extended outwardly at the opposite sidefrom the upstream-side of the input shaft 220.

The PTO clutch mechanism 600A includes a driving-side member 620 whichis supported on the PTO shaft 610 in a relatively rotatable manner andis operatively connected to the input shaft 220, a driven-side member630 which is supported on the PTO shaft 610 in a relativelynon-rotatable manner, and friction plate unit 640 including driving-sidefriction plates supported by the driving-side member 620 in a relativelynon-rotatable manner and driven-side friction plates supported by thedriven-side member 630 in a relatively non-rotatable manner, wherein thefriction plate unit 640 can be selectively engaged or disengaged withthe effect of the hydraulic pressure to engage or disengage the powertransmission from the input shaft 220 to the PTO shaft 610.

The transmission mechanism 280 is configured to be capable oftransferring the rotational power of the input shaft 220 to the firstpump shaft 310 a, the second pump shaft 310 b and the driving-sidemember 620, as illustrated in FIG. 2 and FIG. 4.

In this embodiment, the transmission mechanism 280 includes a first gear281 supported on the input shaft 220 in a relatively non-rotatablemanner, a second gear 282 supported on the second pump shaft 310 b in arelatively non-rotatable manner with engaged with the first gear 281,and a third gear 283 provided on the driving-side member 620 such thatit is engaged with the second gear 282.

As illustrated in FIG. 1, FIG. 2 and FIG. 4, the pump unit 100 includes,in addition to the aforementioned structures, the auxiliary pump mainbody 510 which is operatively driven by the input shaft 220, and anauxiliary pump case 520 connected to the outer surface of the port block360 such that it surrounds the auxiliary pump main body 510.

In this embodiment, the auxiliary pump main body 510 is driven by theoutwardly extended portion of the second pump shaft 310 b, as previouslydescribed.

The pump unit 100 is configured to be capable of supplying fluiddischarged from the auxiliary pump main body 510 to the pair of firsthydraulic fluid lines 400 a and the pair of second hydraulic fluid lines400 b as charge fluid, and also capable of supplying the dischargedfluid to the PTO clutch mechanism 600A as operation fluid.

More specifically, as illustrated in FIG. 1, the pump unit 100 includesa suction line 550 which has a first end fluidly connected to a fluidreservoir (the internal space of the housing 200 in this embodiment) anda second end fluidly connected to a suction port of the auxiliary pumpmain body 510, a discharge line 560 which is fluidly connected to thedischarge port of the auxiliary pump main body 510, a charge line 420which supplies a part of the hydraulic fluid from the discharge line 560to the pair of the first hydraulic fluid lines 400 a and the pair ofsecond hydraulic fluid lines 400 b through a check valve 425, a PTOhydraulic fluid line 470 which supplies a part of the hydraulic fluidfrom the discharge line 560 to the PTO clutch mechanism 600A through aPTO relief valve 478, a pressure-reducing valve 530 which has a primaryside fluidly connected to the discharge line 560, and a resistance valve540 which is fluidly connected to the primary side of thepressure-reducing valve 530.

Further, the PTO hydraulic fluid line 470 and the charge line 420 arefluidly connected to the primary side and a secondary side of thepressure-reducing valve 530, respectively, to enable maintaining thehydraulic pressure in the charge line 420 constant regardless of whetheror not the PTO clutch mechanism 600A is engaged or disengaged, thuspreventing the malfunction of fluctuations in the operation torques ofthe output-adjusting members 350 a and 350 b of the first and secondhydraulic pump main bodies 300 a and 300 b due to the change of thestate of the hydraulic clutch mechanism 600A, which has beenconventionally occurred.

Namely, in a conventional pump unit, the charge line and the PTOhydraulic fluid line are fluidly connected to the secondary side and thespring chamber of the pressure-reducing valve, respectively. Thisconfiguration induces the malfunction of a hydraulic pressure differencein the charge line between when the PTO clutch mechanism is engaged andwhen it is disengaged.

More specifically, when the PTO clutch mechanism is engaged, the PTOhydraulic fluid line is maintained at a predetermined hydraulic pressuredefined by a PTO relief valve which is inserted into the PTO hydraulicfluid line, while when the PTO clutch mechanism is disengaged, the PTOhydraulic fluid line is at a natural pressure since it is drained to thefluid reservoir.

Consequently, in a conventional pump unit, when the PTO clutch mechanismis disengaged, the charge line is maintained at a predeterminedhydraulic pressure with a hydraulic pressure setting spring of thepressure-reducing valve, while when the PTO clutch mechanism is engaged,the charge line is maintained at a pressure which is equal to thebiasing force of the hydraulic pressure setting spring of thepressure-reducing valve plus the hydraulic pressure set by the PTOrelief valve, since the hydraulic pressure of the PTO relief valve actson the spring chamber of the pressure-reducing valve.

Such hydraulic pressure fluctuations in the charge line induce backpressures acting on the first piston unit 330 a, thus causingfluctuations in the operation torques of the output-adjusting members inthe hydraulic pump main bodies. Particularly, when the PTO clutchmechanism is engaged, the hydraulic pressure in the charge line rises,which causes an increase of the hydraulic pressure in the pair ofhydraulic fluid lines, thus increasing the operation torques of theoutput-adjusting members. This offers discomfort to the operator and,for example, in a case where there is provided a neutral positionreturning mechanism for automatically returning the output-adjustingmember to a neutral position in association with the release of theoperating force, the returning to the neutral position will require atime or will not be completely attained in some cases due to theincrease of the operation torque.

On the contrary, in this embodiment, as previously described, the chargeline 420 is fluidly connected to the secondary side of thepressure-reducing valve 530 and the PTO hydraulic fluid sine 470 isfluidly connected to the primary side of the pressure-reducing valve530.

Consequently, even in the event of fluctuations in the hydraulicpressure in the PTO hydraulic fluid line 470 in association with thechange of the state of the PTO clutch mechanism 600A, the charge line420 is maintained at a predetermined hydraulic pressure defined by thehydraulic pressure setting spring of the pressure-reducing valve 530.Therefore, regardless of whether or not the PTO clutch mechanism 600A isactuated, the operation torques of the output-adjusting members 340 aand 340 b in the first and second hydraulic pressure pump main bodies300 a and 300 b can be maintained constant, thus effectively preventingthe aforementioned malfunction which has been occurred in conventionalpump units.

Further, in this embodiment, as illustrated in FIG. 1, the PTO hydraulicfluid line 470 is fluidly connected to the primary side of thepressure-reducing valve 530 through an orifice 472, and also surplusfluid in the discharge line 560 is discharged to the outside of thesystem through the resistance valve 540.

Preferably, there may be provided a drain line 570 for fluidlycommunicating the secondary side of the resistance valve 540 to thefluid reservoir, and an oil cooler 575 may be inserted in the drain line570 (see FIG. 1).

By providing these structures, it is possible to effectively prevent thetemperature rise in stored fluid in the fluid reservoir.

More preferably, there may be provided an external hydraulic fluidextraction line 580 for extracting hydraulic fluid from the drain line570 to outside. The external hydraulic fluid extraction line 580 isconnected to, for example, a hydraulic actuator for lifting andlowering, with respect to the ground surface, a ground working machine(for example, a grass cutting mower machine) which is mounted on theworking vehicle 1 and driven by the PTO shaft 610.

In this embodiment, the pump unit 100 includes the external hydraulicfluid extraction line 580 which has a first end fluidly connected to thedrain line 570, an outside hydraulic fluid return line 581 which has afirst end fluidly connected to the drain line 570, and a switching valve582 for actuating the hydraulic actuator, the switching valve 582 beinginserted in the drain line 570 between the connecting point 580T of theexternal hydraulic fluid extraction line 580 and the drain line 570 andthe connecting point 581T of the outside hydraulic fluid return line 581and the drain line 570.

Further, the outside hydraulic fluid return line 581 is connected to thedrain line 570 at the upstream side of the oil cooler 575, thus enablingeffectively preventing the temperature rise in the fluid in the fluidreservoir.

In this embodiment, the pump unit 100 includes an external hydraulicpressure relief valve 595 which has a primary side fluidly connected tothe discharge line 560, the external hydraulic pressure relief valve 595being for setting the hydraulic pressure in the external hydraulic fluidextraction line 580.

Preferably, as illustrated in FIG. 1, there may be provided a returnline 590 which has a first end fluidly connected to the discharge line560 and a second end fluidly connected to the suction line 550, and theexternal hydraulic pressure relief valve 595 may be inserted in thisreturn line 590.

The concrete structures of the aforementioned respective hydraulicpressure lines will be described.

As illustrated in FIG. 8, in the port block 360, there are formed a pairof first hydraulic fluid passages 410 a forming a part of the pair offirst hydraulic fluid lines 400 a and a pair of second hydraulic fluidpassages 410 b forming a part of the pair of second hydraulic fluidlines 400 b.

The pair of the first hydraulic fluid passages 410 a and the pair ofsecond hydraulic fluid passages 410 b have substantially the samestructure. Consequently, detailed description of the pair of the secondhydraulic fluid passages 410 b will be properly omitted by replacing theend reference character “a” of the pair of the first hydraulic fluidpassages 410 a with “b”.

Each of the pair of the first hydraulic fluid passages 410 a includes akidney port serving as a fluid communication port to the correspondingfirst hydraulic pump main body 300 a and a hydraulic fluid port 411 awhich is opened into the outer surface to serve as a fluid communicationport to the first hydraulic motor unit 10 a.

In this embodiment, as illustrated in FIG. 8, the pair of firsthydraulic fluid passages 410 a are formed in the port block such 360that their first ends and second ends are opened to the outer surface,wherein the first ends form the hydraulic fluid ports 411 a and themiddle portions form the kidney ports.

Out of the pair of the first hydraulic fluid passages 410 a, theforward-movement low-pressure side first hydraulic fluid passage 410a(2), which is brought into a low pressure during forward movement ofthe vehicle, is so configured that the second end is closed by a plug412. On the other hand, the forward-movement high-pressure side firsthydraulic fluid passage 410 a(1), which is brought into a high pressureduring forward movement of the vehicle, is so configured that the secondend is sealed with a relief valve 415 which will be described later.

Specifically, out of the pair of first hydraulic fluid passages 410 aand the pair of second hydraulic fluid passages 410 b, theforward-movement high-pressure side first hydraulic fluid passage 410a(1) and the forward-movement high-pressure side second hydraulic fluidpassage 410 b(1), which are in high pressure state during forwardmovement, are provided with a relief mechanism which releases thehydraulic pressures in the forward-movement high-pressure side firsthydraulic fluid passage 410 a(1) and the forward-movement high-pressureside second hydraulic fluid passage 410 b(1) to the forward-movementlow-pressure side first hydraulic fluid passage 410 a(2) and theforward-movement low-pressure side second hydraulic fluid passage 410b(2), respectively, when the hydraulic pressures in the forward-movementhigh-pressure side first hydraulic fluid passage 410 a(1) and theforward-movement high-pressure side second hydraulic fluid passage 410b(1) exceed a predetermined hydraulic pressure.

In this embodiment, as illustrated in FIG. 4 and FIG. 8, the reliefmechanism includes relief fluid passages 413 formed in the port block360 so as to have first ends fluidly communicated with the second end ofthe forward-movement high-pressure side hydraulic fluid passage 410 a(1)and 410 b(1) and second ends opened into the surface abutting thehousing main body 210, and the relief valves 415 mounted in the portblock 360 so as to be positioned between the forward-movementhigh-pressure side hydraulic fluid passage 410 a(1) and 410 b(1) and thecorresponding relief fluid passages 413.

Further, in this embodiment, in order to enable easily coping withspecification changes, there are further formed, in the port block 360,relief fluid passages 413 corresponding to the forward-movementlow-pressure hydraulic fluid passages 410 a(2) and 410 b(2). The plugs412 are adapted to intercept between the relief fluid passages 413 andthe corresponding forward-movement low-pressure side hydraulic fluidpassages 410 a(1) and 410 b(2).

As illustrated in FIG. 8, the relief valves 415 include a valve housing416 including a valve case having an opening communicated to theforward-movement high-pressure side hydraulic fluid passage 410 a(1) or410 b(1) and a cap which is detachably mounted on the valve case, avalve main body 417 which is slidably accommodated within the valvehousing 416, the valve main body 417 being capable of being positionedat a shutting-off position for closing the opening and a communicatingposition for causing the opening to be opened, and a relief spring 418for biasing the valve main body 417 towards the shutting-off position.

The relief valves 415 act as follows. When the hydraulic pressures inthe forward-movement high-pressure side hydraulic fluid passages 410a(1) and 410 b(1) are equal to or less than a predetermined hydraulicpressure defined by the biasing force of the relief springs 418, therelief valves 415 intercept between the forward-movement high-pressureside hydraulic fluid passages 410 a(1) and 410 b(1) and the relief fluidpassages 413. Also, when the hydraulic pressures in the forward-movementhigh-pressure side hydraulic fluid passages 410 a(1) and 410 b(1) exceedthe predetermined hydraulic pressure, the relief valves 415 cause theforward-movement high-pressure side 410 a(1) and 410 b(1) to becommunicated with the relief fluid passages 413. Consequently, in theevent that the hydraulic motor units 10 a and 10 b temporarily becomeincapable of rotating during forward movement, it is possible to preventthe occurrence of abnormal high pressures in the forward-movementhigh-pressure side hydraulic fluid passages 410 a(1) and 410 b(1) toprotect the first and second hydraulic pump main bodies 300 a and 300 b.

More specifically, as illustrated in FIG. 8, the valve housing 416 maybe provided with a pressure maintaining chamber 416C which slidablysupports the base end portion of the valve main body 417, the pressuremaintaining chamber 416C being separated from the relief fluid passage413. Further, the valve main body 417 may be provided with an axial hole417H between the tip end portion and the base end portion thereof, theaxial hole 417H having an orifice.

By providing these structures, it is possible to slow the movement ofthe valve main body 417 from the shutting-off position to thecommunicating position and the movement thereof from the communicatingposition to the shutting-off position, thereby effectively preventingdamping and/or chattering.

Further, while in this embodiment the relief valves 415 are insertedonly between the forward-movement high-pressure side hydraulic fluidpassages 410 a(1) and 410 b(1) and the corresponding relief fluidpassages 413, and the plugs 412 are employed to intercept between theforward-movement low-pressure side hydraulic fluid passages 410 a(2) and410 b(2) and the corresponding relief fluid passages 413 as previouslydescribed, it is possible to mount the relief valves 415 between theforward-movement low-pressure side hydraulic fluid passages 410 a(2) and410 b(2) and the corresponding relief fluid passages 413. With theconfiguration, even in the event that the hydraulic pressure motor units10 a and 10 b temporarily become incapable of rotating during rearwardmovement, it is possible to prevent the occurrence of abnormal highpressures in the forward-movement low-pressure side hydraulic fluidpassages 410 a(2) and 410 b(2) to protect the first and second hydraulicpump main bodies 300 a and 300 b.

Further, while in this embodiment, the relief valves 415 having thepressure maintaining chambers 416C are employed as previously described,it is possible to insert a relief valve 415B having no such a pressuremaintaining chamber 416C between the forward-movement high-pressure sidehydraulic fluid passage 410 a(1) and the relief fluid passage 413 asillustrated in FIG. 9.

This modified configuration illustrated in FIG. 9 can slow the movementof the valve main body 417 between the shutting-off position and thecommunicating position, while reducing the manufacturing cost.

In the configuration illustrated in FIG. 9, relief valves 415B′ arefurther provided in the forward-movement low-pressure side hydraulicfluid passages 410 a(2) and 410 b(2).

The relief valve main bodies 417′ of the relief valves 415B′ include noaxial hole 417H with an orifice.

Further, as illustrated in FIG. 8, there are formed, in the port block360, a pair of bypass fluid passages 430 for communicating the pair offirst hydraulic fluid passages 410 a to each other and for communicatingthe pair of second hydraulic fluid passages 410 b to each other.

Further, switching valves 436 for opening/closing the pair of bypassfluid passages 430 are mounted in the port block 360.

By providing these bypass fluid passages 430 and the switching valves435, it is possible to prevent the occurrence of hydraulic pressuredifferences between the pair of first hydraulic fluid lines 400 a andbetween the pair of second hydraulic fluid lines 400 b resulting inresistances, when the vehicle is forcibly towed.

FIG. 10 illustrates a cross-sectional view of the auxiliary pump case520 taken along a line X-X in FIG. 2. In FIG. 10, there are illustrateddifferent cross sections.

FIG. 11 illustrates a cross-sectional view taken along a line XI-XI inFIG. 10.

As illustrated in FIG. 1, FIG. 2 and FIG. 10, in the auxiliary pump case520, there is formed a suction fluid passage 551 which has a first endopened to the outer surface to form a suction port 550P and a second endfluidly connected to the suction port of the auxiliary pump main body510.

The suction port 550P is fluidly connected to an external tank 900,which is placed separately from the housing 300, through an externalconduit 552 having a filter 553 inserted therein (see FIG. 1).

In this embodiment, the external conduit 552 and the suction fluidpassage 551 form the suction line 550.

The external tank 900 is connected to the inside space of the housing200 such that fluid can be freely communicated therebetween via acommunication port 200P formed on the wall 213 of the housing 200 (seeFIG. 1 and FIG. 6) and an external conduit 910 connected to thecommunication port 200P. Namely, in this embodiment, the inside space ofthe housing 200 and the external tank 900 form the fluid reservoir.

Further, as illustrated in FIG. 1, FIG. 4 and FIG. 10, in the auxiliarypump case 520, there are formed a suction fluid passage 561 which has afirst end fluidly connected to the discharge port of the auxiliary pumpmain body 510 to form the discharge line 560, a return fluid passage 591(see FIG. 10) which has a first end fluidly connected to the dischargefluid passage 561 and a second end fluidly connected to the suctionfluid passage 551, and the external hydraulic pressure relief valve 595(see FIG. 10) inserted in the return fluid passage 591.

Further, as illustrated in FIG. 10, the auxiliary pump case 520incorporates the pressure-reducing valve 530 such that the primary sidethereof is fluidly connected to the discharge fluid passage 561 and alsoincorporates the resistance valve 540 such that it is fluidly connectedto the primary side of the pressure-reducing valve 530.

Further, in the auxiliary pump case 520, there is formed anauxiliary-pump-case side common charge fluid passage 521 which has afirst end fluidly connected to the secondary side of thepressure-reducing valve 530 and has a second end opened to the surfaceabutting the port block 360, wherein the secondary-pump-case side commoncharge fluid passage 521 forms a part of the charge line 520 (see FIG.10 and FIG. 11).

More specifically, in this embodiment, the charge line 520 includes theauxiliary-pump-case side common charge fluid passage 521 formed in theauxiliary pump case 520 (see FIG. 10 and FIG. 11), a port block sidecharge fluid passage 522 which is formed in the port block 360 such thatit has a first end fluidly connected to the auxiliary-pump-case sidecommon charge fluid passage 521 and a second end opened to the surfaceabutting the first end-face portion 211 of the housing main body 210(see FIG. 5, FIG. 8 and FIG. 11), a charge fluid groove 523 formed inthe outer surface (the surface abutting the port block 360) of the firstend-face portion 211 of the housing main body 210 such that it isfluidly connected to the port block side common charge fluid passage 522(see FIG. 5 and FIG. 11), a pair of first charge fluid passages 524 aformed in the port block 360 such that they have first ends fluidlyconnected to the charge fluid groove 523 and second ends fluidlyconnected to the pair of first hydraulic fluid passages 410 a (see FIG.5 and FIG. 8), a pair of second charge fluid passages 524 b formed inthe port block 360 such that they have first ends fluidly connected tothe charge fluid groove 523 and second ends fluidly connected to thepair of second hydraulic fluid passages 410 b (see FIG. 5 and FIG. 8),and check valves 425 inserted in the pair of first charge fluid passages524 a and the pair of second charge fluid passages 524 b for allowingfluid to flow from the corresponding charge fluid passages 524 a and 524b into the corresponding hydraulic fluid passages 410 a and 410 b andfor preventing the reverse flow thereof (see FIG. 1 and FIG. 4).

In this embodiment, between the forward-movement low-pressure sidehydraulic fluid passages 410 a(2), 410 b(2) and the charge fluidpassages 524 b, orifices are respectively provided in parallel with thecheck valve 425 to prevent the reduction of the transmission efficientlyduring forward movement, while increasing the neutral ranges of thefirst HST and the second HST (see FIG. 1).

These orifices may be provided either in the valve cases for therearward-side check valves 425 or in the plugs 412 as illustrated inFIG. 14. The latter configuration enables utilizing common valve casesfor both the forward and rearward-side check valves 425 and also enableseliminating assembly errors.

Further, in this embodiment, the second end portions of the relief fluidpassages 413 are opened into the charge fluid groove 543 (see FIG. 4 andFIG. 5).

Namely, when the pressures in the forward-movement high-pressure sidehydraulic fluid passages 410 a(1) and 410 b(b) become abnormally high,the hydraulic fluid flows from the forward-movement high-pressure sidehydraulic fluid passages 410 a(1) and 410 b(b) into the forward-movementlow-pressure side hydraulic fluid passages 410 a(2) and 410 b(2) throughthe relief fluid passages 413, the charge fluid groove 523 and thecharge fluid passages 524.

Further, as illustrated in FIG. 1, in addition to the aforementionedhydraulic lines, the pump unit 100 according to this embodiment includesa self-suction line 440 which has a first end fluidly connected to theaforementioned fluid reservoir (the internal space of the housing 200)and a second end fluidly connected to the charge line 420, wherein thereis inserted therein a check valve 445 which allows fluid to flow fromthe fluid reservoir to the charge line 420 while preventing the reverseflow thereof.

In the event that any one of the pair of first hydraulic fluid lines 400a or any one of second hydraulic fluid lines 400 b is brought into anegative pressure during standstill of the auxiliary pump main body 510,the self-suction line 440 sucks fluid from the fluid reservoir into thehydraulic fluid line at such a negative pressure.

Namely, for example, when the working vehicle is parked at a slope orthe like and the engine 40 is stopped with the HST being at a neutralstate, a rotational force is applied to the motor shaft which isoperatively connected to the driving wheel 40, thereby the hydraulicmotor units 10 a and 10 b attempting to perform pumping action.

At this time, when the pair of first hydraulic fluid lines 400 a and thepair of second hydraulic fluid line 400 b are filled with hydraulicfluid, braking forces are acted on the hydraulic motor units 10 a and 10b through the hydraulic fluid, while the pressures in one of the pair offirst hydraulic fluid lines 400 a and one of the pair of secondhydraulic fluid lines 400 b are raised to a high pressure with thepumping action of these hydraulic motor units 10 a and 10 b, thuscausing leakage of hydraulic fluid from the high-pressure hydraulicfluid line.

In the event of the occurrence of hydraulic fluid leakage, the fluid iscirculated from the negative-pressure side hydraulic fluid line to thehigh-pressure side hydraulic fluid line, in the pair of first hydraulicfluid lines 400 a and the pair of second hydraulic fluid lines 400 b,thus facilitating hydraulic fluid leakage from the high-pressure sidehydraulic fluid line. Finally, the hydraulic fluid is removed from thepair of first hydraulic fluid lines 400 a and the pair of secondhydraulic fluid lines 400 b, which causes the driving wheels to freelyrotate and thus causes the vehicle to start descending the slope (freewheel phenomenon).

In view of this point, the pump unit according to this embodimentincludes the self-suction line 440 so that, in the event that any one ofthe pair of first hydraulic fluid lines 400 a or any one of secondhydraulic fluid line 400 b is brought into a negative-pressure, fluid issupplied from the fluid reservoir (the inside space of the housing 200in the illustrated embodiment) to the negative-pressure side hydraulicfluid line, thus effectively preventing the aforementioned free wheelphenomenon.

More specifically, as illustrated in FIG. 11, the self-suction line 440includes a first self-suction fluid passage 441 formed in the port block360 such that it has a first end opened to the inside space of thehousing 200 and a second end opened to the surface abutting theauxiliary pump case 520, and a second self-suction fluid passage 442formed in the auxiliary pump case 520 such that it has a first endopened to the surface abutting the port block 360 to be fluidlyconnected to the first self-suction fluid passage 441 and a second endcommunicated with the auxiliary-pump-case side common charge fluidpassage 521.

Further, the check valve 445 is mounted on the auxiliary pump case 520so as to allow fluid to flow only in the direction from the firstself-section fluid passage 441 to the second self-suction fluid passage442 through the surface abutting (contacting) the port block 360.

FIG. 12 is a cross-sectional view taken along a line XII-XII in FIG. 10.

Further, as illustrated in FIG. 4, FIG. 10 and FIG. 12, the auxiliarypump case 520 is provided with a first PTO hydraulic fluid passage 471 awhich has a first end fluidly connected to the primary side of thepressure-reducing valve 530 and a second end opened to the surfaceabutting the port block 360, an orifice 472 inserted in the first PTOhydraulic fluid path 471, a PTO hydraulic fluid groove 473 which isformed at the surface abutting the port block 360 so as to be fluidlyconnected to the first PTO hydraulic fluid passage, a solenoid valve474, a second PTO hydraulic fluid passage 471 b which has a first endfluidly connected to the PTO hydraulic fluid groove 473 and a second endfluidly connected to the primary side of the solenoid valve 474, a thirdPTO hydraulic fluid passage 471 c which has a first end fluidlyconnected to the secondary side of the solenoid valve 474 and a secondend opened to the surface abutting the port block 360 at a positionspaced apart from the PTO hydraulic fluid groove 473, and a PTO drainfluid passage 650 a which has a first end fluidly connected to the drainport of the solenoid valve 474 and a second end opened to the surfaceabutting the port block 360.

Further, the first PTO hydraulic fluid passage 471 a, the throttle 472,the PTO hydraulic fluid groove 473, the solenoid valve 474, the secondPTO hydraulic fluid passage 471 b and the third PTO hydraulic fluidpassage 471 c form a part of the PTO hydraulic fluid line 470 (see FIG.1).

The PTO drain fluid passage 650 a is opened into the internal space ofthe housing 200 via a PTO drain fluid passage 650 b formed in the portblock 360 (see FIG. 12).

As illustrated in FIG. 7, FIG. 10 and FIG. 12, the PTO hydraulic fluidline 470 further includes a fourth PTO hydraulic fluid passage 471 dformed in the port block 360 such that it has a first end fluidlyconnected to the downstream end portion of the third PTO hydraulic fluidpassage 471 c and a second end opened to the surface abutting the firstend-face portion 211 of the housing main body 210, a fifth PTO hydraulicfluid passage 471 e formed in the first end-face portion 211 such thatit has a first end fluidly connected to the fourth PTO hydraulic fluidpassage 471 d and a second end opened to the internal space of thehousing 200, and an accumulator 660 incorporated in the first end-faceportion 211 such that it is inserted in the fifth PTO hydraulic fluidpassage 471 e, the accumulator 660 including an inlet port 660 a forfluidly connecting a hydraulic chamber of the accumulator 660 to thefifth PTO hydraulic fluid passage 471 e. Further, the PTO hydraulicfluid line 470 includes a PTO internal conduit 475 provided within theinternal space of the housing 200 such that it has a first end fluidlyconnected to the downstream end portion of the fifth PTO hydraulic fluidpassage 471 e and a second portion reaching the inner surface of thecover member 250, a sixth PTO hydraulic fluid passage 471 f formed inthe cover member 250 such that it has a first end fluidly connected tothe downstream end portion of the PTO internal conduit 475 and a secondend opened to an inner surface of a bearing hole which supports the PTOshaft 610 in a bearing manner, and a seventh PTO hydraulic fluid passage471 g formed in the PTO shaft 610 such that it has a first end opened tothe outer surface within the bearing hole and a second end opened to thePTO clutch mechanism 600A. Further, the PTO hydraulic fluid line 470includes a rotary joint 476 formed between the inner surface of thebearing hole and the outer surface of the PTO shaft 610 such that itfluidly connects the sixth PTO hydraulic fluid passage 471 f to theseventh PTO hydraulic fluid passage 471 f, a PTO relief fluid passage477 formed in the cover member 250 such that it has a first end fluidlyconnected to the rotary joint 476 and a second end opened to theinternal space of the housing 200, and the PTO relief valve 478 insertedin the PTO relief fluid passage 477.

As previously described, in the pump unit 100 according to thisembodiment, the accumulator 660 for gradually raising the hydraulicpressure in the PTO hydraulic fluid line 470 to a predeterminedhydraulic pressure defined by the PTO relief valve 478 is incorporatedin the housing 200 which accommodates the first hydraulic pump main body300 a, the second hydraulic pump main body 300 b and the PTO clutchmechanism 600A. This facilitates the efficiency of the piping operation,the prevention of leakage of hydraulic fluid and the reduction of thesize of the entire apparatus.

More specifically, as illustrated in FIG. 12, in this embodiment, theportion of the first end-face portion 211 other then the portion forsupporting the PTO shaft 610 (above the portion for supporting the PTOshaft 610, in this embodiment) is made to be a thick portion.

Further, as illustrated in FIG. 5 and FIG. 10, the accumulator 660 isincorporated within the thick portion of the first end-face portion 211such that its axis is substantially orthogonal to the rotation axes ofthe first and second hydraulic pump main bodies 300 a and 300 b.

Instead thereof, as illustrated in FIG. 13, the accumulator 660 may beincorporated within the wall 213 of the housing main body 210 such thatits axis is substantially parallel to the rotation axes of the first andsecond hydraulic pump main bodies 300 a and 300 b.

Namely, in the embodiment illustrated in FIG. 13, the accumulator 660 isincorporated within the wall 213, and there is formed a fluid groove 665which fluidly communicates the forth PTO hydraulic fluid passage 471 dor the fifth PTO hydraulic fluid passage 471 e to the inlet port 660 aof the accumulator 660, at the surface of the first end-face portion 211abutting the port block 360.

This configuration facilitates the efficiency of the piping operation,the prevention of leakage of hydraulic fluid and the reduction of thesize of the entire apparatus, similarly to this embodiment.

Further, as illustrated in FIG. 4, FIG. 10 and FIG. 11, the auxiliarypump case 520 is provided with the resistance valve 540 which is fluidlyconnected to the primary side of the pressure-reducing valve 530, adrain fluid passage 571 which has a first end fluidly connected to thesecondary side of the resistance valve 540 and has a second end openedto the outer surface to form an outlet port 570Pout, and a first returnfluid passage 651 which has a first end fluidly connected to a springchamber of the resistance valve 540 and a second end opened to thesurface abutting the port block 360.

Further, as illustrated in FIG. 11, the first return fluid passage 651is opened into the fluid reservoir (the internal space of the housing200 in this embodiment) through a second return fluid passage 652 formedin the port block 360.

The drain fluid passage 571 forms a part of the drain line 570.

More specifically, as illustrated in FIG. 1, in addition to the drainfluid passage 571, the drain line 570 includes a drain external conduit572 which has a first end fluidly connected to the drain fluid passage571 via the drain port 570P and a second end fluidly connected to theinternal space of the housing 200 via an inlet port 570Pin (see FIG. 2)formed in the wall 213 of the housing main body 210.

In this embodiment, the external hydraulic fluid extraction line 580 andthe outside hydraulic fluid return line 581 are connected to the drainexternal conduit 572.

Further, the oil cooler 575 is inserted in the drain external conduit572.

Further, while in this embodiment there has been described the dual-typepump unit including the first and second hydraulic pump main bodies 300a and 300 b accommodated within the housing 200, it is obvious that thepresent invention is not limited to this embodiment.

Namely, the present invention may be applied to a single-type pump unitincluding a single hydraulic pump main body within a housing and alsomay be applied to a hydrostatic transmission including a hydraulic pumpmain body and a hydraulic motor main body accommodated within a housing.

Embodiment 2

Hereinafter, another preferred embodiment according to the presentinvention will be described with reference to the attached drawings.

A pump unit 100B according to this embodiment is applied to a vehiclesuch that it forms a part of the travel-system transmission path. Thepump unit is configured to be capable of transmitting a rotational powerfrom a driving power source that is operatively connected thereto tofirst and second hydraulic motor units 10 a and 10 b which drive leftand right driving wheels 50 through a pair of first hydraulic fluidlines 400 a and a pair of second hydraulic fluid lines 400 b.

The components identical or corresponding to those in the aforementionedfirst embodiment will be designated by the identical referencecharacters and description thereof will be properly omitted.

FIG. 15 illustrates a hydraulic circuit diagram of a vehicle to whichthe pump unit 100B according to this embodiment is applied.

More specifically, the pump unit 100B includes the input shaft 220 whichis operatively connected to the driving power source, the first andsecond hydraulic pump main bodies 300 a and 300 b which are operativelydriven by the input shaft 220 and are fluidly connected to the first andsecond hydraulic motor units 10 a and 10 b through the pair of firsthydraulic fluid lines 400 a and the pair of second hydraulic fluid lines400 b, a housing main body (not shown) accommodating the first andsecond hydraulic pump main bodies 300 a and 300 b, and a port block 360B(see FIG. 16) which abuts and supports the first and second hydraulicpump main bodies 300 a and 300 b and defines the accommodating space forthe first and second hydraulic pump main bodies 300 a and 300 b, incooperation with the housing main body.

More specifically, the first hydraulic pump main body 300 a forms afirst HST in cooperation with the first hydraulic motor main body in thefirst hydraulic motor unit 10 a while the second hydraulic pump mainbody 300 b forms a second HST in cooperation with the second hydraulicmotor main body in the second hydraulic motor unit 10 b.

Namely, at least one of the first hydraulic pump main body 300 a and thefirst hydraulic motor main body is of a variable displacement type whichvaries the suction/discharge rate on the basis of the operation of theoutput-adjusting member.

Similarly, at least one of the second hydraulic pump main body 300 b andthe second hydraulic motor main body is of a variable displacement typewhich varies the suction/discharge rate on the basis of the operation ofthe output-adjusting member.

Through the operation of the respective output-adjusting members in thefirst HST and the second HST, the left and right driving wheels can beindividually driven in a variable-speed manner.

As illustrated in FIG. 15, the pump unit 100B further includes acommunication line 100 having an orifice 1100 inserted therein, thecommunication line 100 being for communicating the forward-movementhigh-pressure side first hydraulic fluid line 400 a(1) out of the pairof the first hydraulic fluid lines 400 a to the forward-movementhigh-pressure side second hydraulic fluid line 400 b(1) out of the pairof second hydraulic fluid lines 400 b, wherein the pressures in theforward-movement high-pressure side first hydraulic fluid line 400 a(1)and the forward-movement high-pressure side second hydraulic fluid line400 b(1) are raised to higher pressures during forward movement. Withthis configuration, it is possible to effectively maintain theindependent transmissions of the first and second HSTs while effectivelypreventing or reducing the occurrence of wandering phenomenon that thevehicle meanders even though the respective output-adjusting members ofthe first and second HSTs are operated such that the first and secondHSTs are brought into the same output state.

Namely, by providing the communication line 1000 with the orifice, it ispossible to maintain the hydraulic pressure difference between theforward-movement high-pressure side first hydraulic fluid line 400 a(1)and the forward-movement high-pressure side second hydraulic fluid line400 b(1) when the first and second HSTs are brought into differentoutput states for rotating the vehicle, and also it is possible toprevent or reduce the occurrence of hydraulic pressure differencesbetween the forward-movement high-pressure side first hydraulic fluidline 400 a(1) and the forward-movement high-pressure side secondhydraulic fluid line 400 b(1) when the first and second HSTs are broughtinto the same output state for moving the vehicle straightly.

In this embodiment, the communication line 1000 includes a communicationfluid passage 1010 formed in the port block 360B.

FIG. 16 illustrates a longitudinal cross-sectional front view of theport block 360B. Further, FIG. 17 illustrates a cross sectional viewtaken along a line XVII-XVII in FIG. 16.

As illustrated in FIG. 16, the port block 360B is provided with a pairof first hydraulic fluid passages 410 a forming a part of the pair offirst hydraulic fluid lines and a pair of second hydraulic fluidpassages 410 b forming a part of the pair of second hydraulic fluidlines 400 b.

The pair of first hydraulic fluid passages 410 a and the pair of secondhydraulic fluid passages 410 b have substantially the same structure.Consequently, detailed description of the pair of second hydraulic fluidpassages 410 b will be properly omitted by replacing the end referencecharacter “a” of the pair of first hydraulic fluid passages 410 a with“b”.

Each of the pair of the first hydraulic fluid passages 410 a includes akidney port forming a fluid communication port to the correspondingfirst hydraulic pump main body 300 a and a hydraulic fluid port 411 awhich is opened to the outer surface to form a fluid communication portto the first hydraulic motor unit 10 a.

In this embodiment, as illustrated in FIG. 16, the pair of firsthydraulic fluid passages 410 a are formed in the port block 360B so asto have first ends and second ends opened to the outer surface, whereinthe first ends form the hydraulic fluid ports 411 a and the middleportions form the kidney ports.

Out of the pair of the first hydraulic fluid passages 410 a, theforward-movement low-pressure side first hydraulic fluid passage 410a(2) is so configured that a plug 412 closes the second end, theforward-movement high-pressure side first hydraulic fluid passage 410a(1) is so configured that a relief valve 415 is mounted at the secondend.

The relief valve 415 is provided in order to flow the hydraulic fluidfrom the forward-movement high-pressure side first hydraulic fluidpassage 410 a(1) to the forward-movement low-pressure side firsthydraulic fluid passage 410 a(2), in the event that the hydraulicpressure in the forward-movement high-pressure side first hydraulicfluid passage 410 a(1) exceeds a predetermined hydraulic pressure,

The relief valves 415 include a valve main body 417 which receives thehydraulic pressures in the corresponding forward-movement high-pressureside hydraulic fluid passages 410 a(1) and 410 b(1), the valve main body417 having an axial hole 417H with an orifice, and a valve housing 416having a pressure maintaining chamber 416C defined by the base endportion of the valve main body 417, the pressure maintaining chamber416C being separated from a relief fluid passage 413 which will bedescribed later.

By providing these structures, it is possible to slow the movement ofthe valve main bodies 417 with respect to the hydraulic pressure changesin the corresponding forward-movement high-pressure side hydraulic fluidpassages 410 a(1) and 410 b(1), thus effectively preventing theoccurrence of damping and/or chattering.

Further, the port block 360B is provided with a pair of bypass fluidpassages 430 for communicating the pair of first hydraulic fluidpassages 410 a to each other and for communicating the pair of secondhydraulic fluid passages 410 b to each other, and a pair of switchingvalves 435 for opening/closing the pair of bypass fluid passages 430.

By providing these bypass fluid passages 430 and the switching valves435, it is possible to prevent the occurrence of hydraulic pressuredifferences between the pair of first hydraulic fluid lines 400 a andbetween the pair of second hydraulic fluid lines 400 b when the vehicleis forcibly towed in the event of engine failures and the like.

Further, as illustrated in FIG. 16 and FIG. 17, the port block 360B isprovided with a pair of first charge fluid passages 524 a for supplyingcharge fluid from an auxiliary pump main body 510 provided in the pumpunit 100B or a hydraulic fluid supplying apparatus such as the auxiliarypump unit placed separately from the pump unit 100B to the pair of firsthydraulic fluid passages 400 a, a pair of second charge fluid passages524 b for supplying charge fluid from the auxiliary pump main body orthe hydraulic fluid supplying apparatus to the pair of second hydraulicfluid passages 400 b, and check valves 425 (see FIG. 15) inserted in thepair of first charge fluid passages 524 a and the pair of second chargefluid passages 524 b.

The reference character 523 in FIG. 17 designates a charge fluid groovefor supplying charge fluid from the auxiliary pump main body or thehydraulic fluid supplying apparatus to the pair of first charge fluidpassages 524 a and the pair of second charge fluid passages 524 b.

The reference character 413 in FIG. 16 and FIG. 17 designates a relieffluid passage for communicating the secondary sides of the relief valves415 to the charge fluid groove 523.

Further, as illustrated in FIG. 16, the communication fluid passage 1010is formed in the port block 360B such that it communicates theforward-movement high-pressure side first hydraulic fluid passage 410a(1) and the forward-movement high-pressure side second hydraulic fluidpassage 410 b(1) to each other.

More specifically, in this embodiment, the pair of first hydraulic fluidpassages 410 a and the pair of second hydraulic fluid passages 410 b areformed to be substantially parallel to each other at substantially thesame position in the thicknesswise direction of the port block 360B.

As illustrated in FIG. 15 and FIG. 16, the communication fluid passage1010 is extended substantially in the direction orthogonal to the pairof first hydraulic fluid passages 410 a and the pair of second hydraulicfluid passages 410 b at a position displaced from the pair of firsthydraulic fluid passages 410 a and the pair of second hydraulic fluidpassages 411 b in the thicknesswise direction of the port block 360B,such that it communicates only the forward-movement high-pressure sidefirst hydraulic fluid passage 410 a and the forward-movementhigh-pressure side second hydraulic fluid passage 410 b(1) to eachother.

While in this embodiment the communication fluid passage 1010 is formedat the same side as the bypass fluid passages 430 with respect to thefirst and second pump shafts 310 a and 310 b, and only a single officeis provided as illustrated in FIG. 16, alternatively, a communicationfluid passage 1010′ may be formed at the opposite side from the bypassfluid passages 430 with respect to the first and second pump shafts 310a and 310 b.

The communication fluid passage 1010′ is provided with throttles at thepositions communicated to both the forward-movement high-pressure sidefirst hydraulic fluid passage 410 a(1) and the forward-movementhigh-pressure side second hydraulic fluid passage 410 b(1).

While in this embodiment the communication line 1000 is formed by thecommunication fluid passage 1010 (or 1010′) formed in the port block360B, the present invention is not limited to this configuration.

For example, as illustrated in FIG. 18, the forward-movementhigh-pressure side first hydraulic fluid passage 410 a(1) and theforward-movement high-pressure side second hydraulic fluid passage 410b(1) may be communicated to each other through an external communicationconduit 1020 connected to the port block 360.

More specifically, in the configuration illustrated in FIG. 18, theexternal communication conduit 1020 is configured to communicate thepressure maintaining chamber 416C of the relief valve 415 correspondingto the forward-movement high-pressure side first hydraulic fluid passage410 a(1) with the pressure maintaining chamber 416C of the relief valve415 corresponding to the forward-movement high-pressure side secondhydraulic fluid passage 410 b(1).

With this configuration, it is possible to reduce the thickness of theport block 360 in comparison with the configuration illustrated in FIG.16 and FIG. 17, and also it is possible to utilize thedamping-chattering preventing orifices provided in the axial holes 417Hin the relief valve main bodies 417 as the orifices 1100 inserted in thecommunication line 1000.

Also, in the configuration in which the communication line 1000 isformed by the external communication conduit 1020, it is possible toprovide an orifice between the external communication conduit 1020, andat least one of the pressure maintaining chamber 416C in the reliefvalve 415 provided in the forward-movement high-pressure side firsthydraulic fluid passage 410 a(1) and the pressure maintaining chamber416C in the relief valve 415 provided in the forward-movementhigh-pressure side second hydraulic fluid passage 410 b(1), instead ofthe orifice provided in the axial hole 417H, as illustrated in FIG. 19.

Further, in the configurations illustrated in FIG. 16, FIG. 18 and FIG.19, the pair of first hydraulic fluid passages 410 a and the pair ofsecond hydraulic fluid passages 410 b are respectively concentrated atone side and the other side when viewed along the axes of the pumpshafts 310 a and 310 b.

Specifically, in the aspects illustrated in FIG. 16, FIG. 18 and FIG.19, between the forward-movement high-pressure side first hydraulicfluid passage 410 a(1) and the forward-movement high-pressure sidesecond hydraulic fluid passage 410 b(1), there is placed one of theforward-movement low-pressure side hydraulic fluid passages (theforward-movement low-pressure side first hydraulic fluid passage 410a(2) in the illustrated aspect).

Alternatively, as illustrated in FIG. 20, a port block 360C can beformed such that the forward-movement high-pressure side first hydraulicfluid passage 410 a(1) is adjacent to the forward-movement high-pressureside second hydraulic fluid passage 410 b(1). This enables easilyforming the communication line 1000 by utilizing holes forming one orboth of the pair of bypass fluid passages 430.

Namely, the forward-movement high-pressure side first hydraulic fluidpassage 410 a(1) and the forward-movement high-pressure side secondhydraulic fluid passage 410 b(1) may be placed adjacent to each other atthe center portion of the port block 360C, while the forward-movementlow-pressure side first hydraulic fluid passage 410 a(2) and theforward-movement low-pressure side second hydraulic fluid passage 410b(2) may be placed at outer positions of the port block 360C. Further,one or both of the pair of bypass fluid passages 430 may be extended attheir inner end portions and the extended fluid passages 1030 may beused to form the communication line 1000 for communicating theforward-movement high-pressure side first hydraulic fluid passage 410a(1) to the forward-movement high-pressure side second hydraulic fluidpassage 410 b(1).

In the aspect illustrated in FIG. 19, the pair of bypass fluid passages430 are placed coaxially and both of the pair of bypass fluid passages430 are extended at their inner end portions, wherein the orifice isformed between the opposite end portions of the pair of extended fluidpassages 1030.

This specification is by no means intended to restrict the presentinvention to the preferred embodiments set forth therein. Variousmodifications to the pump unit, as well as the hydrostatic transmissionas described herein, may be made by those skilled in the art withoutdeparting from the spirit and scope of the present invention as definedin the appended claims.

1. A pump unit for vehicle traveling configured to be capable oftransmitting a rotational power through a pair of first hydraulic fluidlines and a pair of second hydraulic fluid lines from a driving powersource which is operatively connected thereto to first and secondhydraulic motor units for driving right and left driving wheels, thepump unit comprising: an input shaft which is operatively connected tothe driving power source; first and second hydraulic pump main bodieswhich are operatively driven by the input shaft and are fluidlyconnected to the first and second motor units through the pair of firsthydraulic fluid lines and the pair of second hydraulic fluid lines; ahousing main body which surrounds the first and second hydraulic pumpmain bodies; a port block which abuts and supports the first and secondhydraulic pump main bodies, and defines the accommodating space for thefirst and second hydraulic pump main bodies in cooperation with thehousing main body; and a communication line which communicates between aforward-movement high-pressure side first hydraulic fluid line, which isbrought into higher pressure during forward movement, out of the pair offirst hydraulic fluid lines and a forward-movement high-pressure sidesecond hydraulic fluid line, which is brought into higher pressureduring forward movement, out of the pair of second hydraulic fluidlines, the communication line having an orifice inserted therein.
 2. Thepump unit according to claim 1, wherein the port block is provided witha pair of first hydraulic fluid passages and a pair of second hydraulicfluid passages which form a part of the pair of first hydraulic fluidlines and the pair of second hydraulic fluid lines, respectively, andthe communication line is configured to communicate between aforward-movement high-pressure side first hydraulic fluid passage, whichis brought into higher pressure during forward movement, out of the pairof first hydraulic fluid passages and a forward-movement high-pressureside second hydraulic fluid passage, which is brought into higherpressure during forward movement, out of the pair of second hydraulicfluid passages.
 3. The pump unit according to claim 2, wherein the pairof first hydraulic fluid passages and the pair of second hydraulic fluidpassages are formed to be substantially parallel to each other, and thecommunication line has a communication fluid passage formed in the portblock such that it is orthogonal to the pair of first hydraulic fluidpassages and the pair of second hydraulic fluid passages.
 4. The pumpunit according to claim 3, wherein the forward-movement high-pressureside first hydraulic fluid passage and the forward-movementhigh-pressure side second hydraulic fluid passage are placed adjacent toeach other.
 5. The pump unit according to claim 2, wherein thecommunication line has a conduit which is detachably connected to theport block such that it communicates the forward-movement high-pressureside first hydraulic fluid passage and the forward-movementhigh-pressure side second hydraulic fluid passage to each other.